Floating base vector sensor

ABSTRACT

Systems and methods are provided for sensing acoustic signals using a floating base vector sensor. A vector sensor according to an embodiment of the present disclosure can be used to detect and characterize low frequency sound wave(s) in a viscous medium (e.g., air, water, etc.) by detecting a periodic motion of the media particles associated with the sound wave(s). The orientation of the particle velocity deduced from such measurements can provide information regarding the wave vector of the sound wave(s), can define the direction of arrival (DOA) for the acoustic signal, and can assist locating the source of the sound of interest.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/970,369, filed on May 3, 2018, which claims the benefit ofU.S. Provisional Patent Application No. 62/500,550, filed on May 3,2017, both of which are incorporated by reference herein in theirentireties.

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The United States Government has ownership rights in this invention.Licensing inquiries may be directed to Office of Technology Transfer atUS Naval Research Laboratory, Code 1004, Washington, D.C. 20375, USA;+1.202.767.7230; techtran@nrl.navy.mil, referencing Navy Case Number102466-US4.

FIELD OF THE DISCLOSURE

This disclosure relates to sensors, including floating sensors.

BACKGROUND

The detection and characterization of sound waves propagating in viscousmediums, such as air or water, is useful for a variety of applications.One factor in detection and characterization of acoustic signals isdetermining the direction of arrival (DOA) of the acoustic signal.Conventional methods for detecting the DOA of acoustic signals haveseveral limitations, especially for low frequency signals. For example,a traditional method of detecting the DOA for underwater acoustics wavesis based on measuring the relative phase or pressure gradient in thesound wave using arrays of spatially separated pressure sensors(hydrophones). The applicability of this multi-sensor method for lowfrequency acoustic signals is limited by the large size of the arrays(monopole separation comparable to the wavelength of the sound) requiredto provide the directionality.

An accelerometer implemented as a neutrally buoyant body immersed in thesound wave underwater provides an alternative established method fordetecting the orientation of the acoustic wave vector that is collinearwith the acceleration of the water particles. The sensitivity of suchaccelerometers diminishes as the frequency decreases, since theacceleration itself drops proportionally to the frequency and thetransducer noise increases as inverse frequency. As a result, the testmass required for operating the accelerometer in 10 Hz frequency rangecan become prohibitively large.

Micromechanical sensors have been recently proposed as vector sensorsfor underwater acoustics. Depending on the geometry of themicromechanical device, the read-out signal (i.e., the deformation ofthe sensor) can be dominated by one of the sound wave components: eitherby the pressure gradient within the sound wave or by the viscous forcesarising from the periodic motion of the surrounding water particles.While promising for applications in very low frequency underwateracoustics, micromechanical vector sensors have two major drawbacks inpractical implementations: (i) the amount of deformation to be measuredis very small and (ii) the sensor is assumed to be somehow rigidlymounted underwater (i.e. the displacement of the flexible, soundsensitive parts of the micromechanical sensor are measured with respectto a rigid base). Both difficulties in achieving an acceptablesignal-noise ratio and the necessity to attach the sensor to a fixedstructure underwater are seen as major obstacles for practicaldeployment of micromechanical vector sensors.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated in and constitute partof the specification, illustrate embodiments of the disclosure and,together with the general description given above and the detaileddescriptions of embodiments given below, serve to explain the principlesof the present disclosure. In the drawings:

FIG. 1 is a diagram of a floating base vector sensor in accordance withan embodiment of the present disclosure;

FIG. 2 shows a diagram of exemplary optional components of an anchor inaccordance with an embodiment of the present disclosure;

FIG. 3 shows diagrams of web geometry in accordance with an embodimentof the present disclosure;

FIG. 4 shows estimates of acoustic drag force for an embodiment of thepresent disclosure;

FIG. 5 is a diagram of an exemplary AVS configured as a sonobuoy with atower moored above an anchor in accordance with an embodiment of thepresent disclosure;

FIG. 6A is a diagram of a cross-section of an exemplary tower with amesh positioned at the throat of a double-horn-shaped cavity inaccordance with an embodiment of the present disclosure;

FIG. 6B is a diagram of a cross-section of an exemplary tower withmembranes separating channels 506 from the surrounding medium inaccordance with an embodiment of the present disclosure; and

FIG. 7 is a diagram of a cross-section of an exemplary tower including apower source, memory, and transmitter in accordance with an embodimentof the present disclosure.

Features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the disclosure. However, it will beapparent to those skilled in the art that the disclosure, includingstructures, systems, and methods, may be practiced without thesespecific details. The description and representation herein are thecommon means used by those experienced or skilled in the art to mosteffectively convey the substance of their work to others skilled in theart. In other instances, well-known methods, procedures, components, andcircuitry have not been described in detail to avoid unnecessarilyobscuring aspects of the disclosure.

References in the specification to “one embodiment,” “an embodiment,”“an exemplary embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

1. Overview

Embodiments of the present disclosure provide systems and methods forsensing acoustic signals that address limitations of prior systems. Forexample, a vector sensor according to an embodiment of the presentdisclosure can be used to detect and characterize low frequency soundwave(s) in a viscous medium (e.g., air, water, etc.) by detecting aperiodic motion of the media particles associated with the soundwave(s). The orientation of the particle velocity deduced from suchmeasurements can provide information regarding the wave vector of thesound wave(s), can define the direction of arrival (DOA) for theacoustic signal, and can assist locating the source of the sound ofinterest.

A vector sensor in accordance with an embodiment of the presentdisclosure can exploit an acoustically-induced normal displacement offine mesh as a measure of the collinear projection of the particlevelocity in the sound wave. In an embodiment, the acoustically inducedflow force on an individual fiber within the mesh is nearly independentof the fiber diameter, and the mesh-flow interaction can bewell-described theoretically by a nearest neighbor couplingapproximation. Scaling arguments based on these two observationsindicate that the refinement of the mesh down to the nanoscale leads tosignificant improvements in performance. The combination of the twodimensional nature of the mesh together with the nano-scale dimensionsprovides a dramatic gain in the total length of fiber exposed to theflow, leading to a sensitivity enhancement by orders of magnitude.

Embodiments of the present disclosure can be used in a variety ofapplications. For example, embodiments of the present disclosure can beused to detect sound waves in water in the vicinity of pressure-releasesurfaces (e.g., a water-air boundary, hull(s) of submerged vessels atlow frequencies, etc.). More generally, a mesh-type velocimeter inaccordance with an embodiment of the present disclosure can be used formonitoring slowly-varying viscous flow down to the DC limit.

2. Low Frequency Acoustic Sensors

Compact, lightweight, low power directional acoustic sensors capable ofoperating at low frequency can be enabling for numerous applications,such as acoustic source localization. In underwater acoustics, wheresub-100 Hz frequency ensures long range propagation for sound waves,such sensors provide the ability to look over narrow angular aperturesthus discriminating against the signals from interfering noise sourcesand providing target bearing information with very small sensorpackages.

A major obstacle in creating such acoustic sensors is the exceedinglysmall value of the sound-induced force. In general, the miniature beamsin these sensors respond at low frequencies to the pressure gradientforce and to the viscous drag force, both associated with the passage ofthe acoustic wave. For example, a 10 mm long slender rod (100 μm OD) ina 100 μPa airborne 100 Hz sound wave (14dBSPL=40 dB(re1 μPa))experiences a total force of less than 1 pN. To provide a measurablemechanical deformation in response to the acoustically-induced flow thecantilever-type structure has to be extremely flexible. The associatedlow bending spring constant then limits the upper frequency that can beattained, and precludes wide bandwidth applications (practical aspectsof handling ultrasoft cantilevers aside).

Embodiments of the present disclosure provide significantly improveddetection levels and higher bandwidths for acoustic flow-based sensorsthat are attainable using micro/nano scale, two dimensional (2D)mesh-based structures, as opposed to essentially macroscale 1Dcantilevers, as the transduction element. In an embodiment, thetransduction element includes a planar mesh structure under significanttension immersed in an acoustic fluid. In an embodiment, the fact thatthe mechanical response of the mesh is controlled by the tensile stress,as opposed to bending rigidity of a cantilever, enables one to tailorthe stiffness of the structure—i.e. to manipulate the resonant frequencyand therefore the bandwidth of the device.

In an embodiment, an incident acoustic plane wavep_(i)e^({right arrow over (k)}√{right arrow over (x)}) induces a forceon each fiber of the mesh in the direction {right arrow over (k)} of theplane wave due to both the pressure gradient across the fiber and theviscous drag force associated with the acoustic velocity field. In anembodiment, the cross section of the fibers comprising the mesh has alength and a width that is that significantly less (e.g., ten timesless) than the viscous penetration depth in the medium for the frequencyof interest.

In an embodiment, for microscale fibers, the viscous drag force isdominant and the pressure gradient force is negligible. The combinationof the extra dimension and the micro/nano scale unit mesh size dprovides a dramatic gain in the total length of the fiber exposed to theacoustically induced flow. In an embodiment, a square L×L filled with afine mesh of unit size d contains the total fiber length 2 L²/d, anincrease by a factor 2 L/d compared to a single cantilever of length L.The total normal force on the mesh can give rise to an out of planedisplacement defined by the out-of-plane stiffness (associated with thetensile stress) of the overall mesh. Tangential forces may result inlocal deformation but do not result in out of plane displacements. Thus,in an embodiment, a measurement of the out-of-plane displacement of themesh is expected to give rise to a natural cos θ (relative to the normalto the mesh) directivity associated with an incident pressure field. Ina prototype of an embodiment of the present disclosure, just such adipole directivity (e.g., as shown in FIG. 3) was observed, showing thatthree co-located orthogonal mesh transducers will allow one toreconstruct the wave vector of a sound wave in 3D space.

In an embodiment, analyzing the multi-scale, fluid-structure interaction(FSI) problem involving long-wavelength acoustic excitation of amicro/nano-scale mesh immersed in a viscous medium can be determinedbased on a theoretical model including an exact solution, fullyincluding viscosity effects, for the acoustic response of twoneighboring filaments. In an embodiment, an approximate solution for theresponse of the mesh can be determined based on a self-consistent,nearest neighbor approximation.

3. Floating Base Vector Sensor

Embodiments of the present disclosure provide a mesh-type (e.g.,web-based) micromechanical structure that is designed to be sensitive toacoustically induced flow and can serve as a particle velocity,directional sensor for underwater or in-air acoustics. A floating basevector sensor in accordance with an embodiment of the present disclosurecan determine the direction of arrival (DOA) for an acoustic signal ofinterest, while featuring a footprint orders of magnitude smaller thanconventional beam-form arrays.

In an embodiment, a fine micro-fabricated web immersed in a viscousmedium undergoes cyclic mechanical deformation (e.g., comparable to thestretching of a trampoline) in the presence of an acoustic excitation.In an embodiment, this deformation of the web can be detected using anoptical probe. In an embodiment, the deformation is governed by theviscous forces due to oscillatory motion of the media particles in thesound wave. The directionality is provided by the fact that theout-of-plane deformation of the mesh strongly depends on the anglebetween the flow direction and the normal vector of the mesh. In anembodiment, by monitoring the deformation of the mesh (e.g., optically),one can extract information regarding both sound intensity and theorientation of the wave vector. In an embodiment, the combination of thetwo-dimensional (2D) nature of the mesh, together with the nano-scaledimensions, provides a dramatic gain in the total length of fiberexposed to the flow, leading to a sensitivity enhancement by orders ofmagnitude. In an embodiment, while strong coupling of the mesh tooscillatory motion of the media particles provides high sensitivity, theability to tailor the resonant frequency of the web through built-intension allows one to shape the frequency response of the device and toextend the bandwidth.

In an embodiment, the applicability of the web-based velocity sensor canbe extended to the detection of sound waves in water in the vicinity ofa pressure-release surface, which is the relevant impedance condition atthe hull of most underwater vessels at low frequencies, e.g. submarinesand autonomous underwater vehicles (AUVs). In an embodiment, a mesh-typevelocity sensor in conjunction with the “floating base” method ofdeployment provides both high sensitivity and the ability to detectsound waves while being suspended in the medium, with no rigid anchoringrequired.

FIG. 1 is a diagram of a floating base vector sensor in accordance withan embodiment of the present disclosure. In FIG. 1, the floating basevector sensor includes a floating base 102, one or more (four shown inFIG. 1) flow meters 104 (e.g., viscous flow meters) attached to floatingbase 102, and a retaining thread 106 coupling floating base 102 to ananchor 108 (e.g., anchored to the ground).

In an embodiment, the floating base vector sensor is operated byinvoking a combination of forces arising from acoustic scattering of theincoming sound wave by some parts of the vector sensor (i.e., floatingbase 102), as well as time-dependent viscous forces applied to otherparts of the sensor (e.g., flow meters 104) by the surrounding waterparticles. In an embodiment, floating base 102 is suspended in water andis free to move in the directions of interest. In an embodiment, thesize and the material of floating base 102 are chosen to maximize theacoustic forces caused by scattering of the incoming sound wave by thebase. In an embodiment, these forces lead to a “recoil” motion offloating base 102 with respect to the media particles, and this relativemotion can be detected by velocity sensor(s) attached to floating base102 and extended far enough to be exposed to the “far field” mediaparticles, unaffected by the recoil.

In an embodiment, the design of a mesh-type velocimeter is governed bytwo key observations: i) the acoustically induced flow force on anindividual fiber within the mesh is nearly independent of the fiberdiameter, and ii) the mesh-flow interaction can be well-described by anearest neighbor coupling approximation. Scaling arguments based onthese two observations indicate that the refinement of the mesh down tothe nanoscale leads to significant improvements in performance. Thecombination of a two dimensional nature of the mesh together with thenano-scale mesh dimensions provides a dramatic gain in total length ofthe fiber exposed to the flow. For example, in an embodiment, a squareL*L filled with a nano-scale mesh with spacing d contains the totalfiber length 2 L²/d, an increase by factor 2 L/d compared to a singlecantilever of length L. As a result, the sensitivity of a floating basevector sensor in accordance with an embodiment of the present disclosurecan be enhanced by at least two orders of magnitude compared tocantilever based designs. In an embodiment, in the presence of a tensilestress, the mechanical response of the mesh is governed by themembrane-type forces (as opposed to bending rigidity in the case ofcantilever-type devices), which provides the ability to tailor theresonant frequency of the web through built-in tension and allows one toshape the frequency response of the device and to extend the bandwidth.

In an embodiment, the “floating base” method of deployment takesadvantage of both acoustic scattering and viscous forces combined. Bychoosing the average density of floating base 102 to be significantlydifferent from the density of water, one can ensure that periodic motionof floating base 102 due to acoustic scattering differs significantlyfrom the motion of the water particles in an unperturbed sound wave.Equation (1) below gives the velocity of the periodic motion of floatingbase 102 due to acoustic scattering effects:

$\begin{matrix}{u_{r} = {\frac{A}{\rho\; c}\left( {{\frac{1}{3}{ika}\frac{\kappa_{b}}{\kappa}} + \frac{3\;\rho}{{2\;\rho_{b}} + \rho}} \right)}} & (1)\end{matrix}$

In Equation (1) above, u_(r) is the motion of the base, c is the speedof sound in the medium, A is the amplitude of the pressure, a is thediameter of the base, k is a wave vector (2π/wavelength) ρ is thedensity of the medium, ρ_(b) is the density of the base, κ iscompressibility, and κ_(b) is compressibility of the base.

In an embodiment, the mesh velocimeter attached to floating base 102 canbe used as a readout for the resulting motion of the base with respectto the surrounding media. In an embodiment, the micromechanicalflow-meter(s) 104 attached to floating base 102 will experience thecorresponding enhancement in the speed of the surrounding fluid flow(compared to a rigidly fixed flow sensor) and therefore improvedsignal/noise ratio.

In one embodiment, floating base 102 can be a sphere made of a lightmaterial (e.g., ρ_(base)«ρ_(water)) that is moored (e.g., free to movein the XY plane) and the flow meters 104 are micromechanical flexiblestructures protruding orthogonally out of floating base 102. In anembodiment, flow meters 104 can be implemented as flexible rods(whiskers), planes (fins), or as meshed membranes that extend fromfloating base 102. In an embodiment, a means of providing the readoutfrom the flow sensor can be rigidly attached to floating base 102. Forexample, for a micromechanical (mesh) sensor, it could be an opticalreadout of the deformation caused by the flow. In an embodiment, as thebuoyancy of floating base 102 is not neutral (by design), floating base102 is linked to anchor 108 that will define the operational depth,while permitting motion of floating base 102 in the directions ofinterest. Both fixed anchors and a floating anchor (with the means ofregulating the depth) can be used with a floating base vector sensor inaccordance with an embodiment of the present disclosure.

In an embodiment, the acoustic scattering force in the floating-basevector sensor is frequency-independent, therefore making the proposedfloating-base vector sensor suitable for low frequency range. In anembodiment, the size of floating base 102 has to be only large enough tooverpower the drag force applied to flow meters 104. Therefore the totalsize of the device can be quite small (an estimate of 10 mm radius for10 Hz operation). The floating base vector sensor can be implemented aslow cost, mass-produced device. In an embodiment, the floating basevector sensor can be deployed by unmanned devices (e.g., AUVs). Theability of the floating base vector sensor to measure the velocity ofthe water particles associated with the sound wave (as opposed topressure) can enable implementations where the floating base vectorsensor is mounted on the hull of the vessel (submarine, AUV, etc.) ormoored in shallow waters, close to an air/water boundary.

A floating base vector sensor envisioned as a positive-buoyancy floatingbase 102 moored on anchor 108 in accordance with an embodiment of thepresent disclosure will be easy to deploy (e.g., in an embodiment,anchor 108 can include a battery, electronics necessary for operatingthe floating base vector sensor, and a means of communication to anexternal device, such as a transceiver, optical communication link,etc.).

FIG. 2 shows a diagram of exemplary optional components of anchor 108 inaccordance with an embodiment of the present disclosure. For example, inan embodiment, anchor 108 is an anchor device configured to communicate(e.g., with an external device 214). In FIG. 2, anchor 108 includes acontroller 202, transmitter 203, battery 204, processor 206, and memory208. It should be understood that these components of anchor 108 areoptional, and all or some components shown in FIG. 2 will notnecessarily be present in every embodiment of the present disclosure.For example, in an embodiment, anchor device 108 may not include aseparate processor 206 or memory 208 and will just include a controller202 (e.g., a controller chip). In some embodiments, anchor 108 caninclude none of the components shown in FIG. 2, and data gathered by thefloating base vector sensor can be gathered by a different device (e.g.,a device attached to, and/or integrated into, floating base 102) forlater collection. In some embodiments, such a device can include atransmitter for communication of this data to an external device 214.

In an embodiment, anchor 108 can receive input data 210. For example, inan embodiment, vector sensor measurement data (e.g., including DOA data)can be communicated to anchor 108. In an embodiment, anchor 108 caninclude a light source coupled to the optical link(s) that deliver lightto flow meters 104 and a receiving link that delivers input data 210from flow meters 104. In an embodiment, this data can (optionally) beprocessed (e.g., using controller 202 and/or processor 206) andcommunicated to an external device 214 (e.g., using transmitter 203, atransceiver, an optical link, etc.). In an embodiment, controller 202and transmitter 203 can also be used to communicate information to thefloating base vector sensor (e.g., to reconfigure components and/orparameters of the floating base vector sensor).

For example, in an embodiment, the fine micro-fabricated webs of flowmeters 104 immersed in a viscous medium, such as water, undergo cyclicmechanical deformation in the presence of an acoustic excitation. In anembodiment, this deformation of the web can be detected using an opticalprobe (e.g., coupled to flow meters 104 and/or to floating base 102). Inan embodiment, by monitoring the deformation of the mesh (e.g.,optically), one can extract information regarding both sound intensityand the orientation of the wave vector. In an embodiment, the opticalprobe is configured to transmit this monitored deformation informationas input data to anchor 108. For example, in an embodiment, the opticallink coupled to flow meters 104 can be run along retaining thread 106 toanchor 108. In an embodiment, anchor 108 can include a velocimeter(e.g., as part of or coupled to controller 202) configured to extractinformation measured by flow meters 104. In an embodiment, one or morevelocimeters can be coupled to floating base 102 configured to extractinformation measured by flow meters 104.

In an embodiment, data can be encrypted before being transmitted asoutput results 212 to external device 214. For example, in anembodiment, controller 202 and/or an encryption module inside, orcoupled to, anchor 108 can be configured to encrypt data before it issent to transmitter 203 for transmission to external device 214.

External device 214 can be a number of devices in accordance withembodiments of the present disclosure. For example, in an embodiment,external device 214 can be a ship, a floating buoy device, a land-basedreceiver, etc. In an embodiment, external device 214 can be a centralcontroller configured to aggregate data from multiple floating basevector sensors. In an embodiment, external device 214 can be configuredto receive data from a single floating base vector sensor. In anembodiment, anchor 108 further includes a receiver configured to receiveinformation from external device 214. In an embodiment, anchor 108(and/or other components of the floating base vector sensor) can beconfigured to decrypt and verify information received from externaldevice 214.

Components of anchor 108 and/or of any other processing device coupledto the floating base vector sensor of FIG. 1 can be implemented usinghardware, software, and/or a combination of hardware and software.Components of anchor 108 and/or of any other processing device coupledto the floating base vector sensor of FIG. 1 can be implemented using asingle device or multiple devices. For example, in an embodiment,components of anchor 108 can be implemented using a single integratedcircuit (chip). In an embodiment, components of anchor 108 and/or of anyother processing device coupled to the floating base vector sensor canbe implemented using a general purpose computer or a special purposedevice configured to communicate with components of the floating basevector sensor of FIG. 1.

4. Fabrication and Experimental Measurements

A “spider web” geometry with 6 mm outer diameter (OD) was chosen as aprototype of the mesh sensor in accordance with an embodiment of thepresent disclosure, fabricated in 1 μm-thick LPCVD (low pressurechemical vapor deposition) grown, ultra-low stress silicon nitride film.A combination of optical lithography and reactive ion etch was used toform the suspended web structure with the support frame carved in theunderlying silicon wafer using deep reactive ion etch. With the filamentseparation reduced to d=20 μm, even such a small area (6 mm OD) canaccommodate a total length of the fibers ≈2.7 meters, a dramaticincrease compared to a 50 mm long cantilever.

Ignoring the coupling between the filaments of the mesh, the(sound-induced) oscillatory drag force acting on a particular filamenthas a weak logarithmic dependence on the fiber diameter for the verysmall Reynold's numbers in typical sound induced flow(Re˜10⁻⁶−10⁻⁹). Thefilaments for the mesh can thus be made extremely fine, 3.6 μm×1 μm inour prototype, with the total drag force on the mesh being proportionalto the total length of the mesh. The normal component of the drag forceproduced by the oscillatory motion of the medium particles causes anout-of-plane deformation of the mesh, akin to the stretching of atrampoline. In an exemplary embodiment, an optical interferometer can beused to detect the oscillatory displacement of a microfabricated mirrorplaced at the center of a ‘spider web’ mesh. In an embodiment, the extraweight associated with the mirror (30 nm-thick aluminum film in ourprototype) is small compared to the mass of silicon nitride mesh and canbe further reduced.

By controlling the tensile stress in the fibers, one can ensure that themechanical response of the mesh is governed by membrane stresses, incontrast to the bending rigidity governed cantilever. A wide range oftunability attainable through the adjustment of the pre-stress is auseful feature in designing the frequency response of the mechanicalsensor. Given the low mass of the mesh, even a minimal amount of tensioncan provide the high resonant frequency (530 Hz as a fundamentalfrequency for our prototype) critical for wide bandwidth devices.

In an embodiment, in-air evaluation of the velocimeter confirmed adipole-type directionality with peak responsivity in excess of 20 nm/Paat 90 Hz (e.g., as shown in FIG. 3). The total force acting on the meshcan be extracted from the measured mirror displacement and the estimatedspring constant of the web. At 90 Hz, the estimated value exceeds theforce that would be acting on a single cantilever (for the same soundintensity) by orders of magnitude, a direct outcome of the increasedfiber length.

FIG. 3 shows diagrams of web geometry in accordance with an embodimentof the present disclosure. Image 302 shows a diagram of web geometryshowing a mirror at the center and beam truncation required to maintaina desired beam separation. Image 304 is an SEM image of the released webwith beam separation 20 μm. Diagram 306 shows directionality of the webwith OD=6 mm and beam separation 20 μm acquired at 90 Hz.

While the demonstrated level of mechanical sensitivity of 20 nm/Pa iscomparable to that of a typical (nondirectional) MEMS microphone, thegeometry of the mesh velocimeter precludes the use of the capacitivereadout and puts an emphasis on optical transduction. Considering thenoise floor imposed by the interferometer ≈2 pm/√{square root over (Hz)}as a major limiting factor, one can estimate a minimum detectable soundpressure in air as MDP_(air)≈100 μPa. In air, the performance of ourfirst mesh prototype is close to that of a “state-of-the-art” heatflow-based “Microflown” velocity sensor (MDP≈20 μPa). In water, theequivalent amount of the drag force would be generated by a sound wavewith the pressure spectral density MDP_(water)≈76 dB(re1 μPa)/√{squareroot over (Hz)}. We find it remarkable that the first prototype wouldproject an MDP close to “Sea State Zero” in shallow waters—SS0≈65 dB(re1μPa)/√{square root over (Hz)} at 100 Hz, level of ambient noise expectedin ocean 180 m below the surface on a calm day.

FIG. 4 shows estimates of acoustic drag force for an embodiment of thepresent disclosure. Specifically, FIG. 4 shows estimates of the acousticdrag force per unit length (in picoNewtons) imposed by a 100 μPaacoustic sound pressure in air (0.24 μm/s flow velocity) in accordancewith an embodiment of the present disclosure. The top plot 402 is for anisolated cylinder. The middle plot 404 is for a linear array of fiberswith separation 20 μm. The bottom plot 406 is for a rectangular meshwith a fiber separation 20 μm. The dimensions for the theoreticalcalculations match the actual dimensions of the “spider web” sensor inaccordance with an embodiment of the present disclosure. Theexperimental data is graphed by plot 408. The results of the numericalsimulations for the corresponding linear 1D array (half-solid circles onplot 404) and mesh (half-solid circles on plot 406) are shown forcomparison. Note that the total force “captured” by a flow sensor iscalculated as a product of the “force per unit length” shown in thefigure and the total length of all the branches in the mesh. Thelogarithmic decrease in the force/length shown in FIG. 4, associatedwith the interaction between the web fibers, is outweighed by increasinglength as the mesh is made finer.

5. Exemplary Alternatives

In an embodiment, a mesh-type micromechanical transducer can be employedas a DC flow meter (velocimeter) with the advantage of small footprintand ability to implement the mesh and optical readout usingcorrosive-resistant materials.

In an embodiment, for operation at an adjustable depth, the floating“base” can be supported by a body with controlled buoyancy (as opposedto an anchor). In such an implementation, the buoyancy of the floatingbase can be chosen as either positive or negative.

In an embodiment, the floating base can have a complicated designoptimized in order to enhance the water flow at the locations of theflow sensors (aka acoustic horn).

In an embodiment, a low symmetry (non-spherical) floating base canprovide additional directional selectivity for detecting sound waves.

In an embodiment, implementations where the floating base has both, highdensity parts and low density parts can enable differential measurementsfor subtracting slow changing water flow (e.g. associated withcurrents).

In an embodiment, the design of the flow sensor attached to the floatingbase can vary. For micromechanical flow sensors, the variation caninclude both the geometry of the sensor (cantilever, plate, porousplate, mesh, etc.) as well as the readout mechanism (optical—intensitybased, interferometric, grating-based, electrical—piezoelectric,piezoresistive, capacitive, etc.).

In an embodiment, with multiple flow sensors oriented in differentdirections in respect to the base, one can implement a higher-ordervector sensor with improved directivity.

In an embodiment, to enhance the coupling between the flow sensor andsurrounding fluid, the entire vector sensor can be placed in ansound-permeable enclosure filled with high viscosity fluid, that isacoustically matched to water.

6. Vector Sensor Tower with Channels Embodiments

A vector sensor in accordance with an embodiment of the presentdisclosure can be implemented using a tower with channels (“tower”). Inan embodiment, the tower can be made using any three-dimensional shape(e.g., cylindrical, spherical, etc.) with channels. For example, in anembodiment, floating base 102 can include channels to become a towerwith channels. In an embodiment, the tower can be detached and floatedto the surface of water so that information can be more easily retrievedfrom the tower. FIG. 5 is a diagram of an exemplary Acoustic VectorSensor (AVS) configured as a sonobuoy with a tower moored above ananchor in accordance with an embodiment of the present disclosure. TheAVS 502 of FIG. 5 includes anchor 108 (e.g., as described above withreference to FIG. 1) and a tower 504. In an embodiment, tower 504 has apositive buoyancy and channels 506 a and 506 b are filled with water.

In an embodiment, AVS 502 can determine the intensity and direction ofan incoming sound wave by detecting a relative displacement of viscousliquid (e.g., water) that fills channels 506 carved in the body of tower504 with respect to tower 504. In an embodiment, such relativedisplacement originates from the dissimilarity in reaction of the liquidand tower 504 itself to forces induced by the sound wave and can bedefined by the difference in the density of tower 504 and that of theliquid. In an embodiment, the sound-induced relative displacement isdetected by flow sensors (e.g., mesh type, cantilever, etc.) that areplaced inside of the cavities of channels 506 and attached to tower 504.In an embodiment, multiple channels within tower 504 may have differentorientations.

The configuration of the cavities of channels 506 and the shape of tower504 (e.g., sphere, cylinder, etc.) can be adjusted to optimize dynamicrange and directionality of AVS 502. For example, in an embodiment,cavities of channels 506 can be shaped as acoustic horns with the flowsensor positioned at the throat to enhance the response of AVS 502. FIG.6A is a diagram of a cross-section of an exemplary tower with a meshpositioned at the throat of a double-horn-shaped cavity in accordancewith an embodiment of the present disclosure. FIG. 6A shows flow sensor602 positioned at the throat of the cavity of channel 506 a to enhancethe response of AVS 502. In an embodiment, each channel 506 can includeone or more flow sensors.

In an embodiment, when a flexible membrane separating channels 506 fromthe surrounding medium is installed, a wide variety of liquids (e.g.,oil, ester, etc.) can be used to fill the inside of channels 506. FIG.6B is a diagram of a cross-section of an exemplary tower with membranesseparating channels 506 from the surrounding medium in accordance withan embodiment of the present disclosure. In FIG. 6B, membranes 604(e.g., made of a flexible material such as rubber) separate channel 506a (and, in an embodiment, the liquid inside channel 506 a) from theenvironment. In an embodiment, the materials of membranes 604 isflexible enough to enable liquid in the surrounding environment (e.g.,water) to push liquid (e.g., oil) in the channel (e.g., channel 506 a)but rigid enough so that steady (e.g., non-oscillatory) motion of tower504 will not harm membranes 604. In an embodiment, choices of values ofdensity and viscosity can be used to optimize the response of AVS 502.

In an embodiment, tower 504 has a negative buoyancy, but channels 506are filled with a liquid that is lighter than the surroundingenvironment (e.g., oil can fill channels 506, which is lighter thanwater). In an embodiment, a condition can be reached when the overalldensity of AVS 502 (e.g., the average of tower 504 and liquid withintower 504) is equal to that of the external medium (e.g., water). In anembodiment, tower 504 can have a neutral buoyancy and yet will generatea differential motion of tower 504 with respect to the liquid withinchannels 506 in response to the acoustic wave.

In an embodiment, tower 504 can have multiple channels filled withliquids of various densities (e.g., channels 506 a and 506 b can befilled with different liquids having different densities). In anembodiment, a differential signal acquired from pairs of flow metersimmersed in these different liquids can provide a way to mitigate theeffects of steady flow (of non-acoustic origin). Applications forneutrally-buoyant AVS embodiments can include hull-mounted sensorsand/or towed arrays.

In an embodiment, a positively-buoyant AVS 502 can be used as acomponent of a sonobuoy. For example, tower 504 can be tethered toanchor 108 that contains a controller 202 for operating flow sensors(e.g., flow sensor 602), a power source (e.g., battery 204), processor(e.g., processor 206), and memory (e.g., memory 208) for recordingoutput of AVS 502 (e.g., output measured by flow sensors, such as flowsensor 602). In an embodiment, anchor 108 is sunk (e.g., to the bottomof a body of water). Alternatively, in an embodiment, the buoyancy ofanchor 108 can be adjusted to maintain a pre-defined depth for AVS 502.

In an embodiment, tower 504 (which can be positively-buoyant) can alsoinclude an additional power source, memory and transmitter. FIG. 7 is adiagram of a cross-section of an exemplary tower including a powersource, memory, and transmitter in accordance with an embodiment of thepresent disclosure. FIG. 7 shows transmitter 702, memory 704, andbattery 706 implemented within tower 504. It should be understood thatthe positions of transmitter 702, memory 704, and battery 706 shown inFIG. 7 is not limiting and that transmitter 702, memory 704, and battery706 can be implemented in any part of tower 504 in accordance withembodiments of the present disclosure. Further, in an embodiment, asecond processor (and/or controller) can also be implemented withintower 504. In an embodiment, retaining thread 106 can be detached (e.g.,at pre-determined conditions, such as after a predetermined time, aftertower 504 has detected a predetermined event, and/or after apredetermined number of observations have been recorded by tower 504),and transmitter 702 can be activated once tower 504 comes to the surfaceto beam out stored information.

7. Conclusion

In accordance with embodiments of the present disclosure, the shift froma 1D to a 2D geometry leads to significant performance gains whileexpanding the bandwidth. In an embodiment, the scaling transformationx→sx (s<1, x denotes the mesh parameters a and d), leads to sensitivityenhancements in proportion to the scale factor s⁻²f(sqd), while thepenalty resulting from the coupling between the mesh fibers (the factorf(sqd)) is weak (logarithmic). In the low frequency limit, theresponsivity of the mesh velocimeter exhibits logarithmic frequencydependence defined by the interplay between the mesh unit size andfrequency-dependent viscous penetration depth. This represents asignificant improvement compared to the 1/ω fall off featured byaccelerometers and opens possibilities for infrasonic applications.Further, we have found that the mesh sensors are robust andstraightforward to fabricate.

It is to be appreciated that the Detailed Description, and not theAbstract, is intended to be used to interpret the claims. The Abstractmay set forth one or more but not all exemplary embodiments of thepresent disclosure as contemplated by the inventor(s), and thus, is notintended to limit the present disclosure and the appended claims in anyway.

The present disclosure has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the disclosure.Thus, the breadth and scope of the present disclosure should not belimited by any of the above-described exemplary embodiments.

What is claimed is:
 1. A vector sensor, comprising: a tower comprising aplurality of flow channels, wherein the tower is configured to detect arelative displacement of viscous liquid that fills the plurality of flowchannels; a plurality of flow sensors places inside respective cavitiesof respective flow channels in the plurality of flow channels, whereinthe plurality of flow sensors are configured to detect the relativedisplacement of the viscous liquid that fills the plurality of flowchannels; a retaining thread coupled to the tower; and an anchor coupledto the retaining thread.
 2. The vector sensor of claim 1, wherein thetower is a floating tower submerged in water, and wherein the anchor isanchored to an ocean floor.
 3. The vector sensor of claim 1, wherein thetower is configured to be detached from the retaining thread, therebyenabling the tower to float to a surface of a body of water.
 4. Thevector sensor of claim 1, wherein the tower is further configured todetermine an intensity and direction of an incoming sound wave bydetecting the relative displacement of viscous liquid that fills theplurality of flow channels.
 5. The vector sensor of claim 1, furthercomprising: a transmitter.
 6. The vector sensor of claim 1, wherein theanchor comprises a controller configured to control the flow sensors. 7.The vector sensor of claim 1, wherein the plurality of flow sensorscomprise two-dimensional meshes.
 8. The vector sensor of claim 1,wherein the plurality of flow channels have different orientations. 9.The vector sensor of claim 8, wherein the orientations of the flowchannels are configured to optimize a dynamic range and directionalityof the vector sensor.
 10. The vector sensor of claim 5, wherein thetower further comprises: a memory; and a battery.
 11. The vector sensorof claim 10, wherein the tower is configured to be detached from theretaining thread, thereby enabling the tower to float to a surface of abody of water, and wherein the transmitter is configured to transmitinformation stored in the memory after the tower has reached the surfaceof the body of water.
 12. A vector sensor, comprising: a towercomprising a plurality of flow channels; a plurality of flow sensorsplaces inside respective cavities of respective flow channels in theplurality of flow channels, wherein the plurality of flow sensors areconfigured to detect a relative displacement of viscous liquid thatfills the plurality of flow channels; a retaining thread coupled to thetower; and an anchor device, coupled to the retaining thread, whereinthe anchor device is configured to: receive an incoming signalcorresponding to the detected displacement of the viscous liquid, andtransmit a signal corresponding to the detected displacement of theviscous liquid to an external device.
 13. The vector sensor of claim 12,wherein the anchor device further comprises: a battery; a transmitter;and a controller.
 14. A vector sensor, comprising: a tower, comprising:a transmitter, a memory, a battery, and a plurality of flow channels; aplurality of flow sensors places inside respective cavities ofrespective flow channels in the plurality of flow channels, wherein theplurality of flow sensors are configured to detect a relativedisplacement of viscous liquid that fills the plurality of flowchannels; a retaining thread coupled to the tower; and an anchor coupledto the retaining thread, wherein the tower is configured to be detachedfrom the anchor by decoupling the tower from the retaining thread,thereby enabling the tower to float to a surface of a body of water, andwherein the transmitter is configured to transmit information stored inthe memory after the tower has reached the surface of the body of water.15. The vector sensor of claim 14, wherein the tower is furtherconfigured to determine an intensity and direction of an incoming soundwave by detecting the relative displacement of viscous liquid that fillsthe plurality of flow channels.
 16. The vector sensor of claim 14,wherein the plurality of flow channels have different orientations. 17.The vector sensor of claim 16, wherein the orientations of the flowchannels are configured to optimize a dynamic range and directionalityof the vector sensor.
 18. The vector sensor of claim 14, wherein thetower is a floating tower submerged in water, and wherein the anchor isanchored to an ocean floor.
 19. The vector sensor of claim 14, whereinthe plurality of flow sensors comprise two-dimensional meshes.
 20. Thevector sensor of claim 14, wherein the anchor further comprises: asecond battery; a second transmitter; and a controller.